Zoom Lens and Imaging Apparatus

ABSTRACT

The zoom lens is composed of: an object side lens group at least including, in order from an object side: a first lens group G 1  having positive refractive power; and a second lens group G 2  having negative refractive power; and an image focusing side lens group including, in order from the object side: a negative lens group A having negative refractive power; and a negative lens group B arranged by facing the negative lens group A across an air distance, having negative refractive power. In the zoom lens, focusing from infinity to a close object is achieved by moving just the negative lens group A toward an image focusing side to satisfy a conditional expression below: 
       [Expression 1] 
       −1.80&lt;β2 t &lt;−0.94  (1)
 
       (1−β At   2 )×β Bt &lt;−4.5  (2)
 
     where “β2t” is lateral magnification at a telephoto end of the second lens group in infinity
 
focusing, “βAt” is lateral magnification at a telephoto end of the negative lens group A in infinity focusing, and “βBt” is lateral magnification at a telephoto end of the negative lens group B in infinity focusing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/284,630, filed on May 22, 2014, and published as U.S. PatentApplication Publication No. 2014/0347524 A1, the disclosure of which ishereby incorporated in its entirety by reference, which claims priorityto Japanese Patent Application No. 2013-108754 filed May 23, 2013, thedisclosure of which is hereby incorporated in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a zoom lens and an imaging apparatusincluding the zoom lens, and more particularly to a small-sized zoomlens having a high magnification change ratio, and an imaging apparatusincluding the zoom lens.

2. Description of the Related Art

Imaging apparatuses including a digital still camera using solid stateimaging sensors have become widespread. In recent years, small-sizedimaging apparatus systems using small-sized solid state imaging sensorsincluding Micro Four Thirds have been rapidly developing. Accordingly,requirement on a miniature zoom lens having high imaging performancehave increased depending on market needs for a zoom lens adjusting afocal length depending on an object as an optical systems in the imagingsystems. Moreover, in recent years, market needs for a telephoto zoomlens with a high magnification change ratio like a focal lengthexceeding 300 mm in terms of 35-mm film have been increased.

In an interchangeable lens for a small-sized imaging system of suchtype, focusing by a contrast method instead of a conventional phasedifference method has been employed. In the phase difference method, afocus position is determined on the basis of distance informationestimated by a phase difference sensor. In contrast, a peak position ofcontrast of a subject image determined on an imaging sensor surface isdetected while a focus lens group is moved along an optical axis todetect the peak position as a focus position in the contrast method. Asdescribed above, in the contrast method, moving of a focus lens group inorder to detect a peak position of contrast is necessary. As a result, afocus speed tends to reduce as compared with the phase differencemethod. Accordingly, to achieve high speed autofocusing in the contrastmethod, high speed movement of a focus lens group is necessary.

A zoom lens disclosed in Japanese Patent Laid-Open No. 2009-265652achieves high imaging performance all over a zooming area by increasingthe number of movable lens groups in zooming to increase degrees offreedom of aberration correction. At the same time, a rear focus methodis applied to reduce a size of a focus lens group in a radial directionto achieve weight reduction of the focus lens group, and high speedautofocusing is achieved.

Problems to be Solved

Since a small-sized imaging apparatus system has a small-sized imagingapparatus body, such a telephoto zoom lens having a high magnificationchange ratio is required to reduce a size in a direction not only anoverall optical length but also a diameter of a lens barrel. However,although the zoom lens disclosed in Japanese Patent Laid-Open No.2009-265652 intends to reduce a diameter of a lens barrel by making arear lens group composed of lenses with relatively small diameters afocus lens group, it is not enough to achieve reduction of a size in aan overall optical length direction. Therefore, further downsizing ofthe zoom lens is required. Further, in the zoom lens disclosed inJapanese Patent Laid-Open No. 2009-265652, as reduction in a movement ofthe focus lens group in focusing is insufficient, higher speedautofocusing is required.

An object of the present invention is to provide a small-sized zoom lenscapable of having high imaging performance and achieving a highmagnification change ratio as well as high speed autofocusing, and toprovide an imaging apparatus including the zoom lens.

SUMMARY OF THE INVENTION

As a diligent study of the present inventors, the object is achieved byapplying a rear focus method to a zoom lens of a telephoto typedescribed below.

The zoom lens according to the present invention is composed of anobject side lens group having positive refractive power and an imagefocusing side lens group having negative refractive power arranged inorder from an object side wherein, the object side lens group at leastincluding a first lens group having positive refractive power and asecond lens group having negative refractive power in order from theobject side, and the image focusing side lens group including a negativelens group A having negative refractive power and a negative lens groupB having negative refractive power arranged by facing the negative lensgroup A across an air distance in order from the object side; and ischaracterized in that focusing from infinity to a close object isachieved by moving just the negative lens group A toward an imagefocusing side and satisfy conditional expressions below.

−1.80<β2t<−0.94  (1)

(1−βAt ²)×βBt ²<−4.5  (2)

where “β2t” is lateral magnification at a telephoto end of the secondlens group in infinity focusing, “βAt” is lateral magnification at atelephoto end of the negative lens group A in infinity focusing, and“βBt” is lateral magnification at a telephoto end of the negative lensgroup B in infinity focusing.

In the zoom lens according to the present invention, it is preferablethat the image focusing side lens group satisfies the conditionalexpression below.

2.1<βrt<3.5  (3)

where “βrt” is composite lateral magnification at a telephoto end of theimage focusing side lens group in infinity focusing.

In the zoom lens according to the present invention, it is preferablethat the negative lens group A satisfies the conditional expressionbelow.

1.05<βAt/βAw<1.45  (4)

where “βAt” is lateral magnification at a telephoto end of the negativelens group A in infinity focusing, and “βAw” is lateral magnification ata wide angle end of the negative lens group A in infinity focusing.

In the zoom lens according to the present invention, it is preferablethat the object side lens group is provided a positive lens group Chaving positive refractive power disposed at most image focusing sidefacing the negative lens group A in the object side lens group across anair distance.

In the zoom lens according to the present invention, it is preferablethat the positive lens group C in the object side lens group and thenegative lens group B of the image focusing side lens group move alongthe same trace in magnification change from the wide angle end to thetelephoto end.

In the zoom lens according to the present invention, it is preferablethat the first lens group satisfies the conditional expression below.

0.5<fl/√{square root over ((fw×ft))}<2.5  (5)

where “fl” is a focal length of the first lens group, “fw” is a focallength of the zoom lens at the wide angle end, and “ft” is a focallength of the zoom lens at the telephoto end.

The imaging apparatus according to the present invention includes thezoom lenses described above, and an imaging sensor which converts anoptical image formed by the zoom lens into an electric signal providedon the image focusing side of the zoom lenses.

Advantages of the Invention

According to the present invention, a small-sized zoom lens capable ofhaving high imaging performance and achieving a high magnificationchange ratio as well as high speed autofocus, and to provide an imagingapparatus including the zoom lens above can be provided by employingrear focus method by applying a telephoto system of so-called atelephoto type to minimize a movement of each of lens groups in zoomingand focusing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of the zoom lens in Example 1showing a structural example of the zoom lens at a wide angle end;

FIG. 2 includes a spherical aberration graph, an astigmatism graph, anda distortion aberration graph, in a state in which the zoom lens inExample 1 at the wide angle end is focused on infinity;

FIG. 3 includes a spherical aberration graph, an astigmatism graph, anda distortion aberration graph, in a state in which the zoom lens inExample 1 at an intermediate focal length is focused on infinity;

FIG. 4 includes a spherical aberration graph, an astigmatism graph, anda distortion aberration graph, in a state in which the zoom lens inExample 1 at a telephoto end is focused on infinity;

FIG. 5 is a schematic sectional view of the zoom lens in Example 2showing a structure of the zoom lens at a wide angle end;

FIG. 6 includes a spherical aberration graph, an astigmatism graph, anda distortion aberration graph, in a state in which the zoom lens inExample 2 at the wide angle end is focused on infinity;

FIG. 7 includes a spherical aberration graph, an astigmatism graph, anda distortion aberration graph, in a state in which the zoom lens inExample 2 at an intermediate focal length is focused on infinity;

FIG. 8 includes a spherical aberration graph, an astigmatism graph, anda distortion aberration graph, in a state in which the zoom lens inExample 2 at a telephoto end is focused on infinity;

FIG. 9 is a schematic sectional view of the zoom lens in Example 3showing a structure of the zoom lens at a wide angle end;

FIG. 10 includes a spherical aberration graph, an astigmatism graph, anda distortion aberration graph, in a state in which the zoom lens inExample 3 at the wide angle end is focused on infinity;

FIG. 11 includes a spherical aberration graph, an astigmatism graph, anda distortion aberration graph, in a state in which the zoom lens inExample 3 at an intermediate focal length is focused on infinity; and

FIG. 12 includes a spherical aberration graph, an astigmatism graph, anda distortion aberration graph, in a state in which the zoom lens inExample 3 at a telephoto end is focused on infinity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the zoom lens and the imaging apparatus according to thepresent invention will be described below.

1. Zoom Lens 1-1. Structure of an Optical System

First, a structure of an optical system of the zoom lens according tothe present invention will be described. The zoom lens according to thepresent invention is a zoom lens of so-called a telephoto type composedof an object side lens group having positive refractive power and animage focusing side lens group having negative refractive power arrangedin order from an object side. As the zoom lens is a telephoto type, anoverall optical length of the zoom lens at the telephoto end can be madeshorter than a focal length of the zoom lens at a telephoto end.Accordingly, even if a magnification change ratio is increased to afocal length exceeding 300 mm in terms of 35 mm film, for example,increase of an overall optical length at a telephoto end can behindered.

In the present invention, the object side lens group at least includes afirst lens group having positive refractive power and a second lensgroup having negative refractive power in order from the object side;and the image focusing side lens group includes a negative lens group Ahaving negative refractive power and a negative lens group B havingnegative refractive power arranged by facing the negative lens group Aacross an air distance in order from the object side. The zoom lens ischaracterized in that just the negative lens group A is moved toward theimage focusing side in focusing from infinity to a close object.

In the present invention, negative refractive power in the imagefocusing side lens group can be easily increased since the zoom lens isa telephoto type as described above and the image focusing side lensgroup includes the negative lens group A having negative refractivepower and the negative lens group B arranged on the image focusing sideby facing the negative lens group A across an air distance. Thus, as azoom lens is easy to a have stronger telephoto tendency, even if amagnification change ratio is increased, overall optical length at atelephoto end can be made shorter than a focal length at the telephotoend.

A zoom lens generally contains one or more inner cylinders in a lensbarrel (most outer cylinder) in a telescopic manner. The inner cylindersare drawn to an object side depending on a magnification change ratio.If a difference in overall optical lengths at a telephoto end and a wideangle end increases, a plurality of inner cylinders should be containedin a most outer cylinder to make an overall length of the lens barrelshort if the inner cylinders are contained. However, containing theplurality of inner cylinders in the most outer cylinder increases adiameter of the most outer cylinder by thicknesses of the innercylinders. Thus, in the present invention, as a zoom lens with astronger telephoto tendency is applied as described above, increase inboth an overall optical length at the telephoto end and the number ofthe inner cylinders to be contained in the most outer cylinder can beprevented even if a magnification change ratio is increased. Accordingto the present invention, not only an outer diameter of the lens barrelbut also the overall optical length at the telephoto end can be reduced.

In the present invention, it is preferable that the object side lensgroup is provided a positive lens group C having positive refractivepower disposed at most image focusing side facing the negative lensgroup A in the object side lens group across an air distance.Arrangement of the positive lens group C having positive refractivepower on the most image focusing side of the object side lens groupenables a focal length of the object side lens group shorter and itmakes an overall optical length of the zoom lens shorter. In addition,as diameter of flux of incident light to the negative lens group A ofthe image focusing side lens group can be reduced by a convergenceaction of the positive lens group C, diameters of lenses constitutingthe image focusing side lens group may be further reduced, i.e. it ispreferable for downsizing in the radial direction.

1-2. Motion

Next, a focusing and a magnification change in a zoom lens of thestructure described above will be described in order.

(1) Focusing

A focusing will be described. In the zoom lens according to the presentinvention, just the negative lens group A acts as a focus lens group asdescribed above, and focusing from infinity to a close object isachieved by moving just the negative lens group A toward an imagefocusing side. As the negative lens group A having relatively small lensdiameters compared with each of lenses constituting an object side lensgroup acts as a focus lens group, not only weight reduction of the focuslens group but also reduced movement of the focus lens group in focusingare achieved. That is, high speed autofocusing is achieved and the zoomlens is downsized.

In order to constitute a zoom lens having a storing telephoto tendency,increase negative refractive power in an image focusing side lens groupis required as described above. In a conventional zoom lens of atelephoto type, refractive power of the negative lens group A has beenset as negative, and refractive power of the negative lens group B hasbeen set as positive, in general. However, if just the negative lensgroup A acts as a focus lens group, and the negative lens group A hasstrong refractive power, aberration fluctuation and a view anglefluctuation occurs depending on movement of the negative lens group A infocusing. Thus, in the present invention, as negative refractive poweris also allocated to the negative lens group B subsequent to thenegative lens group A, a zoom lens with a strong telephoto tendency asdescribed above is constituted and the aberration fluctuation and theview angle fluctuation in focusing is prevented while preventing thenegative refractive power of the negative lens group A from becoming toostrong. In an imaging apparatus not provided an optical finder includinga mirror-less interchangeable-lens camera, a user performs focusingwhile viewing a live-view image in a liquid crystal display provided ona back surface of an apparatus body. In this case, if the zoom lensaccording to the present invention is used, an image with high imagingperformance as a live-view image can be displayed while preventingmagnification change action in focusing. Accordingly, the zoom lensaccording to the present invention is suitably applicable to an imagingapparatus such as a mirror-less interchangeable-lens camera.

In addition, if the positive lens group C described above is provided ata most image focusing side of the object side lens group, furtherreduction of a diameter of flux of incident light to the negative lensgroup A can be achieved depending on a convergence action of thepositive lens group C, a diameter of each of lenses constituting thenegative lens group A can be further reduced. Accordingly, furtherincreased autofocusing and further downsizing in the zoom lens isachieved.

(2) Magnification Change

A magnification change will be described. In the zoom lens according tothe present invention, although an motion of each of lens groups is notespecially limited in magnification change, it is preferable in view ofachieving high imaging performance all over a zooming area by improvinga degree of freedom of aberration correction that each of the lensgroups relatively move in magnification change for distances changebetween each of the lens groups. Because distance changes between eachof the lens groups in magnification change makes adjustment to apreferable position in aberration correction of each of the lens groupsin each of magnification change ratios easy. Note that, distance changebetween each of the lens groups may be achieved by moving individuallyeach of all the lens groups in magnification change or moving integrallysome lens groups of all the lens groups and separately residual lensgroups. In addition, all the lens groups may not be a movable lensgroup, i.e. some lens groups may be a fixed lens group.

In view of increasing freedom in aberration correction, it is preferableto individually move each of all the lens groups in magnificationchange. However, in view of manufacturing, it is preferable in thepresent invention that the positive lens group C in the object side lensgroup and the negative lens group B in the image focusing side lensgroup are integrally moved along the same trace in magnification change.If the positive lens group C and the negative lens group B arranged backand forth of the negative lens group A acts as the focus lens groupintegrally move, two lens groups can be manufactured as one unit andmanufacturing efficiency is improved and assembling error is prevented.As a result, a lens movement mechanism is simplified compared with acase where the positive lens group C and the negative lens group B aremoved separately. In addition, a guide shaft for guiding movement of thenegative lens group A can be held from both ends by a lens holding framefor holding each of lens in the positive lens group C and a lens holdingframe for holding each of lens in the negative lens group B since thepositive lens group C and the negative lens group B are unitized.Accordingly, hold of the guide shaft parallel to an optical axis is madeeasy to move the negative lens group A stably and it prevent image blur.

In addition, it is preferable in the present invention that whenmagnification is changed from a wide angle end to a telephoto end, thenegative lens group A moves toward the image focusing side with respectto the positive lens group C temporarily, and then moves toward theobject side. As such motion of the negative lens group A inmagnification change enables a distances change between the positivelens group C and the negative lens group A; and the negative lens groupA and the negative lens group B depending on a magnification changeratio even if the positive lens group C and the negative lens group Bmove along the same trace, it is preferable for aberration correction.

Since the zoom lens according to the present invention described aboveis one aspect of the zoom lens according to the present invention, aspecific lens structure or the like may be appropriately arranged withina range without departing from the essence of the present invention. Inaddition, although there is no detailed description above, to furthershorten an overall optical length of the zoom lens and to furtherdownsize the zoom lens in a radial direction in the present invention, alens group having positive refractive power may be provided between thepositive lens group C and a second lens group in the object side lensgroup.

1-3. Conditional Expression

Conditional expressions which the zoom lens according to the presentinvention should satisfy, or is preferable to be satisfied, will bedescribed. The zoom lens according to the present invention shouldsatisfy conditional expressions (1) and (2) described below, and it ispreferable to satisfy conditional expressions (3) to (5) describedlater.

−1.80<β2t<−0.94  (1)

(1−βAt ²)×βBt ²<−4.5  (2)

where “β2t” is lateral magnification at a telephoto end of the secondlens group in infinityfocusing, “βAt” is lateral magnification at a telephoto end of thenegative lens group A in infinity focusing, and “βBt” is lateralmagnification at a telephoto end of the negative lens group B ininfinity focusing.

1-3-1. Conditional Expression (1)

First, the conditional expression (1) will be described. The conditionalexpression (1) specifies lateral magnification at a telephoto end of thesecond lens group in the zoom lens according to the present invention.As satisfaction of the conditional expression (1) makes lateralmagnification at the telephoto end of the second lens group a propervalue, the overall optical length at the telephoto end and aberrationcorrection can be properly adjust. If magnification is equal to or lessthan a lower limit value in the conditional expression (1), the lateralmagnification of the second lens group is too large, and many lenses arerequired for aberration correction to achieve high imaging performance.As a result, the overall optical length at the telephoto end increases.In contrast, if magnification is equal to or more than an upper limitvalue in the conditional expression (1), the lateral magnification ofthe second lens group is too small, the lateral magnification in theimage focusing side lens group should be increased to achieve a highmagnification change ratio. As a result, many lenses are required foraberration correction to achieve high imaging performance. That is, thenumber of lenses constituting an optical system of the zoom lensincrease and overall optical length increases. As described above,magnification of out of the range of the conditional expression (1) isnot preferable since downsizing of the zoom lens in either case is madedifficult.

In view of the matters above, it is preferable that the conditionalexpression (1) satisfies a condition below to achieve the effect above.

−1.60<β2t<−0.94  (1)′

In addition, it is more preferable that the conditional expression (1)satisfies a condition below to achieve the effect described above.

−1.50<β2t<−0.94  (1)″

1-3-2. Conditional Expression (2)

Next, the conditional expression (2) will be described. The conditionalexpression (2) specifies focus sensitivity at a telephoto end of thenegative lens group A. Satisfaction of the conditional expression (2)makes focus sensitivity in a telescopic motion a proper value and amovement of the negative lens group A in focusing to be in a properrange. If magnification is equal to or more than an upper limit value inthe conditional expression (2) is not preferable for downsizing of thezoom lens since the focus sensitivity is too small and a movement of thenegative lens group A in focusing increases.

1-3-3. Conditional Expression (3)

In the zoom lens according to the present invention, it is preferablethat the image focusing side lens group satisfies the conditionalexpression (3) below.

2.1<βrt<3.5  (3)

where “βrt” is composite lateral magnification at a telephoto end of theimage focusing side lens group in infinity focusing.

The conditional expression (3) specifies lateral magnification of theimage focusing side lens group in the zoom lens according to the presentinvention. The image focusing side lens group includes the negative lensgroup A and the negative lens group B as described above.

Satisfaction of the conditional expression (3) achieves a downsized zoomlens with high magnification change ratio and a high imagingperformance. In addition, satisfaction of the conditional expression (3)secures a proper flange back required for an imaging apparatus includinga mirror-less interchangeable-lens camera.

If magnification is equal to or less than a lower limit value in theconditional expression (3), lateral magnification in the image focusingside lens group is too small and lateral magnification in an object sidelens group should be large to achieve a high magnification change ratio.As a result, a diameter of each of lenses constituting the object sidelens group should be large to make downsizing the zoom lens in a radialdirection difficult. Also, reduction of an overall optical length ismade difficult. In contrast, if magnification is equal to or more thanan upper limit value in the conditional expression (3), the lateralmagnification in the image focusing side lens group is too large andmany lenses are required for aberration correction to achieve highimaging performance. That is, the number of lenses constituting anoptical system of the zoom lens increase to increase the overall opticallength. As described above, magnification of out of the range of theconditional expression (3) is not preferable since downsizing of thezoom lens is made difficult in either case.

In view of the matters above, it is more preferable that the conditionalexpression (3) satisfies a condition below to obtain the effect above.

2.2<βrt<3.5  (3)′

In addition, it is more preferable that the conditional expression (3)satisfies a condition below to obtain the effect described above.

2.3<βrt<3.5  (3)″

1-3-4. Conditional Expression (4)

In the zoom lens according to the present invention, it is preferablethat the negative lens group A satisfies the conditional expression (4)below.

1.05<βAt/βAw<1.45  (4)

where “βAt” is lateral magnification at a telephoto end of the negativelens group A in infinity focusing, and “βAw” is lateral magnification ata wide angle end of the negative lens group A in infinity focusing.

The conditional expression (4) specifies a magnification change ratio ofthe negative lens group. A. In the zoom lens according to the presentinvention, the negative lens group A acts as a focus lens group asdescribed above, and just the negative lens group A moves in focusing.Satisfaction of the conditional expression (4) makes magnificationchange ratios from the wide angle end to the telephoto end in thenegative lens group A in a proper range, to make control of aberrationfluctuation and view angle fluctuation in focusing described above easy.As a result, the effect described above can be enhanced by making thenegative lens group A act as the focus lens group.

1-3-5. Conditional Expression (5)

Next, the conditional expression (5) will be described. In the zoom lensaccording to the present invention, it is preferable that the first lensgroup satisfies the conditional expression (5) below.

0.5<fl/√{square root over ((fw×ft))}<2.5  (5)

where “fl” is a focal length of the first lens group, “fw” is a focallength of the zoom lens at the wide angle end, and “ft” is a focallength of the zoom lens at the telephoto end.

The conditional expression (5) specifies a focal length of the firstlens group. As satisfaction of the conditional expression (5) makes amovement of the first lens group in magnification change in a properrange, high imaging performance while, preventing an increase in thenumber of lenses for aberration correction is achieved. So, it ispreferable for downsizing of the zoom lens.

If the value is equal to or less than a lower limit value in theconditional expression (5), refractive power of the first lens group istoo large and axial color aberration at the telephoto end deteriorates.As a result, many lenses are required for aberration correction toachieve high imaging performance. In view of downsizing the zoom lens,it is not preferable that increased number of lenses increase theoverall optical length. In contrast, if the value is equal to or morethan an upper limit value in the conditional expression (5), refractivepower of the first lens group is too small and a movement of the firstlens group in magnification change increases. As a result, a differencein overall optical lengths at the wide angle end and the telephoto endincreases. If so, the number of inner cylinders to be contained in anouter cylinder increase or a mechanism of drawing the inner cylinders ismade complicate since the difference in overall optical lengths at thewide angle end and the telephoto end increases. That is, it is notpreferable since a lens barrel structure is made complicate to increasean outer diameter of the lens barrel.

In view of the matters above, it is more preferable that the conditionalexpression (5) satisfies a condition below to achieve the effect above.

0.6<fl/√(fw×ft)<2.2  (5)′

In addition, it is more preferable that the conditional expression (5)satisfies a condition below to obtain the effect described above.

0.7<fl/√(fw×ft)<2.0  (5)″

2. Imaging Apparatus

Next, the imaging apparatus according to the present invention will bedescribed. The imaging apparatus according to the present inventionincludes the zoom lens described above, and an imaging sensor whichconverts an optical image formed by the zoom lens into an electricsignal provided on the image focusing side of the zoom lenses. There isno specific limitation on the imaging sensor. However, as describedabove, the zoom lens is suitable for an imaging apparatus of a type notprovided an optical finder and/or a reflex mirror since a flange back ofthe zoom lens according to the present invention can be made short. Inparticular, as the zoom lens according to the present invention is smalland can achieves a high magnification change ratio, therefore, theimaging apparatus is preferable to act as a small-sized imagingapparatus provided with a small-sized solid state imaging sensorincluding so-called mirror-less interchangeable-lens camera.

The present invention will be specifically described by showing Examplesand Comparative Examples. The present invention is not limited to theExamples below, and lens structures disclosed in the Examples below justexemplify the zoom lens according to the present invention. So, the lensstructure of the zoom lens according to the present invention can beappropriately arranged without departing from the essence of the presentinvention.

Example 1 (1) Example of a Lens Structure of a Zoom Lens

FIG. 1 shows Example of a lens structure of a zoom lens in Example 1. Asshown in FIG. 1, the zoom lens in the Example 1 includes: in order froman object side, a first lens group G1 having positive refractive power;a second lens group G2 having negative refractive power; a third lensgroup G3 acts as a positive lens group C having positive refractivepower; a fourth lens group G4 acts as a negative lens group A havingnegative refractive power; and a fifth lens group G5 acts as a negativelens group B having negative refractive power. The first lens group G1to the third lens group G3 constitute the object side lens group, andthe fourth lens group G4 and the fifth lens group G5 constitute theimage focusing side lens group.

The first lens group G1 includes: in order from the object side, acemented lens in which a meniscus lens L1 provided with a convex facingthe object side, having a negative refractive power, and a lens L2having positive refractive power, are cemented; and a meniscus lens L3provided with a convex facing the object side, having a positiverefractive power. The second lens group G2 includes: in order from theobject side, a meniscus lens L4 provided on its object side with anaspherical surface and on its image focusing side with a concave havinga large curvature, the meniscus lens L4 having negative refractivepower; a biconcave lens L5; a biconvex lens L6; and a meniscus lens L7provided with a concave facing the object side, having negativerefractive power. The third lens group G3 includes: in order from theobject side, a biconvex lens L8 provided on its each of both sides withan aspherical surface; a biconcave lens L9; and biconvex lens L10. Theforth lens group G4 includes: a cemented lens in which, in order fromthe object side, a biconvex lens L11 and a biconcave lens L12 providedon its image focusing side with an aspherical surface are cemented. Thefifth lens group G5 includes: a meniscus lens L13 provided with aconcave facing the object side, having negative refractive power; and ameniscus lens L14 provided with a convex facing the image focusing side,having positive refractive power.

In the zoom lens in the Example 1 having the structure above, each ofthe lens groups moves as follows in magnification change from a wideangle end to a telephoto end as shown in FIG. 1 with an arrow: the firstlens group G1 moves toward the object side; the second lens group G2moves toward the image focusing side by drawing a convex trace; thethird lens group G3 moves toward the object side; the fourth lens groupG4 moves toward the image focusing side with respect to the third lensgroup G3 by drawing a convex trace; and the fifth lens group G5 movestoward the object side. In addition, the fourth lens group G4 movestoward the image focusing side in focusing from infinity to a closeobject.

(2) Examples of Numeric Values

In the Example 1, lens data of Example 1 of numeric values to whichspecific numeric values are applied is shown in Table 1. The lens datashown in Table 1 is as follows: “NS” denotes a face number of a lens anddenotes order of lens surfaces from the object side; “R” denotes acurvature radius of a lens surface; “D” denotes a distance between lenssurfaces adjacent to each other along an optical axis; “Nd” denotes arefractive index with respect to a d-line (wavelength λ of 587.6 nm);and “νd” denotes an Abbe number with respect to the d-line (wavelength λof 587.6 nm). In addition, a diaphragm is denoted as a character of “S”in FIG. 1. In Table 1, “STOP” is denoted as a face number of thediaphragm (opening diaphragm). If a lens surface is an asphericalsurface, “ASPH” is designated as a face number, and a paraxial curvatureradius is shown in a section of the curvature radius “R”.

TABLE 1 NS R D Nd νd  1 64.9819 1.3000 1.90366 31.31  2 36.3975 0.01001.56732 42.84  3 36.3975 6.6600 1.49700 81.61  4 −1186.1757 0.2000  534.2934 4.2232 1.61800 63.39  6 162.5347 D (6)  7 ASPH 33.6698 0.20001.51460 49.96  8 36.8067 0.8000 1.91082 35.25  9 8.1262 4.0531 10−29.8667 0.6500 1.91082 35.25 11 20.0064 0.4000 12 15.8824 2.98021.92286 20.88 13 −31.7119 0.7663 14 −16.6818 0.6000 1.77250 49.62 15−54.0405 D (15) 16 STOP 0.0000 1.2000 17 ASPH 9.0025 3.2330 1.5831359.46 18 ASPH −17.0238 0.4600 19 −52.2330 0.5000 1.90366 31.31 2012.6447 1.5345 21 46.2818 2.9182 1.59282 68.62 22 −9.5695 D (22) 23100.3805 1.2000 1.80518 25.46 24 −28.6956 0.0100 1.56732 42.84 25−28.6956 0.6000 1.80139 45.45 26 ASPH 19.7020 D (26) 27 −10.7494 0.63001.80518 25.46 28 −17.3803 0.2000 29 −4854.1028 2.1691 1.48749 70.44 30−20.5041 D (30) 31 0.0000 9.8000 32 0.0000 2.8000 1.51680 64.20 330.0000 1.0000

Table 2 shows aspherical surface coefficients and conic constants if ashape of an aspherical surface shown in Table 1 is expressed by thefollowing expression X(y).

X(y)=(y ² /R)/[1+(1−ε·y ² /R ²)^(1/2) ]+A4·y ⁴ +A6·y ⁶ +A8·y ⁸ +A10·y ¹⁰

In the expression above, “X(y)” denotes a distance (sagging amount) froma peak of each of aspherical surfaces at a height y from the opticalaxis in a vertical direction, along the optical axis direction, and “R”denotes a curvature radius (paraxial curvature radius) of a referencesphere surface, “ε” designating a conic coefficient, and each of “A4,A6, A8, and A10” denote an aspherical surface coefficient.

TABLE 2 ASPH ε A4 A6 A8 A10 7 1.0000 −1.81150e−006 −3.53409e−0072.30973e−009 −1.22024e−011 17 1.0000 −1.28660e−004 1.17974e−006−4.72888e−008 −2.76128e−009 18 1.0000 4.39407e−004 1.33550e−006−1.82741e−007 0.00000e+000 26 1.0000 −2.01216e−005 −1.13690e−0061.04261e−007 −2.22909e−009

Table 3 shows a surface distance in each of states of a wide angle end(f=10.31), an intermediate focal length (f=41.50), and a telephoto end(f=100.60) in the Example 1 of numeric values as well as a focal length(f), an F-number (F-No.), and a field angle (ω) in each of the states.

TABLE 3 F 10.31 41.50 100.60 F-No. 3.657 5.267 5.799 ω 40.1947 10.91384.5726 D (6) 0.9310 19.9590 33.2042 D (15) 19.0512 4.6009 1.6230 D (22)1.9788 3.7822 0.5120 D (26) 7.0763 5.2729 8.5431 D (30) 0.0000 13.788420.1450

Table 4 shows a surface distance in focusing at an close object in eachof states of a wide angle end (f=10.31), an intermediate focal length(f=41.50), and a telephoto end (f=100.60) in the Example 1 of numericvalues as well as a focal length (f) in focusing at an infinite object,and a distance (D(0)) from a first lens surface to an object in each ofthe states.

TABLE 4 f 10.31 41.50 100.60 D (0) 919.86 901.49 884.90 D (22) 2.04304.3047 2.5769 D (26) 7.0121 4.7504 6.4782

FIG. 2 shows spherical aberration, astigmatism, and distortionaberration, in focusing at infinity of the zoom lens in Example 1 ofnumeric values above in a wide angle end. FIG. 3 shows sphericalaberration, astigmatism, and distortion aberration, in focusing atinfinity of the zoom lens in an intermediate focal length. FIG. 4 showsspherical aberration, astigmatism, and distortion aberration, infocusing at infinity of the zoom lens at a telephoto end.

Example 2 (1) Example of a Structure of a Zoom Lens

FIG. 5 shows Example of a lens structure of a zoom lens in the Example2. As shown in FIG. 5, the zoom lens in the Example 2 includes: in orderfrom an object side, a first lens group G1 having positive refractivepower; a second lens group G2 having negative refractive power; a thirdlens group G3 acts as a positive lens group C having positive refractivepower; a fourth lens group G4 acts as a negative lens group A havingnegative refractive power; and a fifth lens group G5 acts as a negativelens group B having negative refractive power. In the Example 2 as wellas the Example 1, the first lens group to the third lens groupconstitute the object side lens group, and the fourth lens group and thefifth lens group constitute the image focusing side lens group.

The first lens group G1 includes: in order from the object side, acemented lens in which a meniscus lens L1 provided with a convex facingthe object side, having a negative refractive power, and a lens L2having positive refractive power, are cemented; and a meniscus lens L3provided with a convex facing the object side, having a positiverefractive power. The second lens group G2 includes: in order from theobject side, a meniscus lens L4 provided on its object side with anaspherical surface and on its image focusing side with a concave havinga large curvature, the meniscus lens L4 having negative refractivepower; a biconcave lens L5; a biconvex lens L6; and a meniscus lens L7provided with a concave facing the object side, having negativerefractive power. The third lens group G3 includes: in order from theobject side, a biconvex lens L8 provided on its each of both sides withan aspherical surface; a biconcave lens L9; and biconvex lens L10. Thefourth lens group G4 includes: a cemented lens in which in order fromthe object side, a biconvex lens L11 and a biconcave lens L12 providedon its image focusing side with an aspherical surface are cemented. Thefifth lens group G5 includes: a meniscus lens L13 provided with aconcave facing the object side, having negative refractive power; and abiconvex lens L14.

In the zoom lens in the Example 2 having the structure above, each ofthe lens groups moves as follows in magnification change from a wideangle end to a telephoto end as shown in FIG. 5 with an arrow: the firstlens group G1 moves toward the object side; the second lens group G2moves toward the image focusing side by drawing a convex trace; thethird lens group G3 moves toward the object side; the fourth lens groupG4 moves toward the image focusing side with respect to the third lensgroup G3 by drawing a convex trace; and the fifth lens group G5 movestoward the object side. In addition, the fourth lens group G4 movestoward the image focusing side in focusing from infinity to a closeobject.

(2) Examples of Numeric Values

In the Example 2, Table 5 shows lens data of Example 2 of numeric valuesto which specific numeric values are applied. The lens data shown inTable 5 is similar to the lens data shown in Table 1.

TABLE 5 NS R D Nd νd  1 65.0172 1.3000 1.91048 31.31  2 36.2100 0.01001.57046 42.84  3 36.2100 6.0000 1.49845 81.61  4 −2179.5150 0.2000  535.2814 4.0027 1.62032 63.39  6 183.6531 D (6)  7 ASPH 42.2125 0.20001.51700 49.96  8 42.6979 0.8000 1.91695 35.25  9 8.4806 4.0102 10−40.2053 0.6500 1.91695 35.25 11 19.8739 0.4000 12 15.7705 2.91081.93323 20.88 13 −39.4484 0.7583 14 −17.4656 0.6000 1.77621 49.62 15−52.0671 D (15) 16 STOP 0.0000 1.2000 17 ASPH 8.5883 3.0750 1.5854759.46 18 ASPH −25.0697 0.4400 19 171.5901 0.5000 1.91048 31.31 2010.4093 1.6207 21 25.6522 3.1313 1.59489 68.62 22 −9.9776 D (22) 2346.2354 1.2000 1.81263 25.46 24 −53.2640 0.0100 1.57046 42.84 25−53.2640 0.6000 1.80558 45.45 26 ASPH 13.2084 D (26) 27 −11.9913 0.63001.81263 25.46 28 −21.7212 0.2000 29 57.2469 2.1490 1.48914 70.44 30−29.7248 D (30) 31 0.0000 2.8000 1.51872 64.20 32 0.0000 1.0000

Table 6 shows aspherical surface coefficients and conic constants of theaspherical surface shown in Table 5 as well as Table 2 shows asphericalsurface coefficients and conic constants.

TABLE 6 ASPH ε A4 A6 A8 A10 7 1.0000 8.18698e−006 −2.73054e−0071.74363e−009 −8.23298e−012 17 1.0000 −1.01823e−004 2.84220e−006−6.99155e−008 −7.96183e−010 18 1.0000 4.60590e−004 3.18830e−006−1.41926e−007 0.00000e+000 26 1.0000 −1.47382e−005 −1.68264e−0061.30906e−007 −2.85225e−009

Next, Table 7 shows a surface distance in each of states of a wide angleend (f=10.30), an intermediate focal length (f=38.91), and a telephotoend (f=100.21) in the Example 2 of numeric values as well as a focallength (f), an F-number (F-No.), and a field angle (ω) in each of thestates.

TABLE 7 F 10.30 38.91 100.21 F-No. 3.6579 5.0177 5.8760 ω 40.250 11.5714.601 D (6) 0.9300 21.0041 33.8012 D (15) 19.9939 5.7856 1.5907 D (22)1.3754 2.5090 0.5000 D (26) 6.4996 5.3660 7.3750 D (30) 9.8031 20.742428.8244

Table 8 shows a surface distance in focusing at an close object in eachof states of a wide angle end (f=10.30), an intermediate focal length(f=38.91), and a telephoto end (f=100.21) in the Example 2 of numericvalues as well as a focal length (f) in focusing at an infinite object,and a distance (D(0)) from a first lens surface to an object in each ofthe states.

TABLE 8 f 10.30 38.91 100.21 D (0) 921.00 904.19 887.51 D (22) 1.41672.8588 1.9523 D (26) 6.4584 5.0162 5.9227

FIG. 6 shows spherical aberration, astigmatism, and distortionaberration, in focusing at infinity of the zoom lens in Example 2 ofnumeric values above in a wide angle end. FIG. 7 shows sphericalaberration, astigmatism, and distortion aberration, in focusing atinfinity of the zoom lens in an intermediate focal length. FIG. 8 showsspherical aberration, astigmatism, and distortion aberration, infocusing at infinity of the zoom lens at a telephoto end.

Example 3 (1) Example of a Structure of a Zoom Lens

FIG. 9 shows Example of a lens structure of a zoom lens in Example 3. Asshown in FIG. 9, the zoom lens in the Example 3 includes: in order froman object side, a first lens group G1 having positive refractive power;a second lens group G2 having negative refractive power; a third lensgroup G3 having positive refractive power; a fourth lens group G4 actsas a positive lens group C having positive refractive power; a fifthlens group G5 acts as a negative lens group A having negative refractivepower; and a sixth lens group G6 acts as a negative lens group B havingnegative refractive power.

The first lens group G1 includes, in order from the object side: acemented lens in which a meniscus lens L1 provided with a convex facingthe object side, having a negative refractive power, and a lens L2having positive refractive power, are cemented; and a lens L3 providedwith a convex facing the object side, having a positive refractivepower. The second lens group G2 includes, in order from the object side:a biconcave lens L4; a meniscus lens L5 provided with a convex facingthe object side, having positive refractive power; and a lens L6provided with a concave facing the object side, having negativerefractive power. The third lens group G3 includes, in order from theobject side: a biconvex lens L7; a biconvex lens L8; a cemented lens inwhich a biconvex lens L9 and a biconcave lens L10 are cemented; and acemented lens in which a biconcave lens L11 and a lens L12 provided witha convex facing the object side, having positive refractive power. Thefourth lens group G4 includes, in order from the object side: a biconvexlens L13; and a cemented lens in which a biconvex lens L14 and abiconcave lens L15 are cemented. The fifth lens group G5 includes acemented lens in which a biconvex lens L16 and a biconcave lens L17 arecemented. The sixth lens group G6 includes a lens L18 provided with aconcave facing the object side, having negative refractive power.

In the zoom lens in the Example 3 having the structure above, each ofthe lens groups moves as follows in magnification change from a wideangle end to a telephoto end as shown in FIG. 9 with an arrow: the firstlens group G1 moves toward the object side; the second lens group G2 actas a fixed lens group and is fixed with respect to the image surface;the third lens group G3 moves toward the object side; the fourth lensgroup G4 moves toward the object side; the fifth lens group G5 movestoward the object side; and the sixth lens group G6 moves toward theobject side. In addition, the fifth lens group G5 moves toward the imagefocusing side in focusing from infinity to a close object.

(2) Examples of Numeric Values

In the Example 3, Table 9 shows lens data of Example 3 of numeric valuesto which specific numeric values are applied.

TABLE 9 NS R D Nd νd  1 249.5578 1.5000 1.83400 37.34  2 76.6801 0.01001.56732 42.84  3 76.6801 5.4040 1.49700 81.61  4 −415.9125 0.1500  574.9171 5.1230 1.48749 70.44  6 −373.1804 D (6)  7 −113.5760 0.80001.74546 49.68  8 18.2990 0.0100 1.56732 42.84  9 18.2990 2.5665 1.8051825.46 10 69.1683 1.9870 11 −41.5554 0.8000 1.80420 46.50 12 1027.6603 D(12) 13 34.0523 3.3030 1.49700 81.61 14 −52.3911 0.1000 15 47.07492.1776 1.48749 70.44 16 −106.4688 0.1000 17 30.8990 3.4205 1.48749 70.4418 −33.1763 0.0100 1.56732 42.84 19 −33.1763 0.8000 1.90888 34.10 2066.1108 6.8000 21 −503.4365 0.7000 1.77554 43.58 22 15.8629 0.01001.56732 42.84 23 15.8629 2.1182 1.90366 31.31 24 40.2035 2.4723 25 STOP0.0000 D (25) 26 54.7228 2.3101 1.60241 37.99 27 −29.1238 0.1000 2821.8055 2.9393 1.52364 53.81 29 −24.6681 0.0100 1.56732 42.84 30−24.6681 0.6750 1.90366 31.31 31 80.8965 D (31) 32 49.9105 1.50001.80518 25.46 33 −60.6167 0.0100 1.56732 42.84 34 −60.6167 0.58801.76157 48.92 35 16.7139 D (35) 36 −25.3286 0.9400 1.48749 70.44 37−95.5860 D (37) 38 0.0000 2.8000 1.51680 64.20 39 0.0000 1.0000

Next, Table 10 shows a surface distance in each of states of a wideangle end (f=72.10), an intermediate focal length (f=148.41), and atelephoto end (f=291.00) in the Example 3 of numeric values as well as afocal length (f), an F-number (F-No.), and a field angle (ω) in each ofthe states.

TABLE 10 F 72.10 148.41 291.00 F-No. 4.2519 4.9044 6.3077 ω 6.488 3.1621.614 D (6) 27.2994 56.7662 66.8379 D (12) 24.1336 17.3427 1.5720 D (25)5.0483 4.0747 6.4009 D (31) 4.4201 1.5040 2.8355 D (35) 11.7641 14.680213.3487 D (37) 18.6600 26.4245 39.8695

Table 11 shows a surface distance in focusing at an close object in eachof states of a wide angle end (f=14.43), an intermediate focal length(f=57.85), and a telephoto end (f=145.40) in the Example 3 of numericvalues as well as a focal length (f) in focusing at an infinite object,and a distance (D(0)) from a first lens surface to an object in each ofthe states.

TABLE 11 F 72.10 148.41 291.00 D (0) 1055.44 1025.97 1015.90 D (31)5.6636 4.7678 11.7660 D (35) 10.5206 11.4164 4.4182

FIG. 10 shows spherical aberration, astigmatism, and distortionaberration, in focusing at infinity of the zoom lens in Example 3 ofnumeric values above in a wide angle end. FIG. 11 shows sphericalaberration, astigmatism, and distortion aberration, in focusing atinfinity of the zoom lens in an intermediate focal length. FIG. 12 showsspherical aberration, astigmatism, and distortion aberration, infocusing at infinity of the zoom lens at a telephoto end.

Table 12 shows values of each of the conditional expressions (1) to (5)above if specific numeric values are applied to the zoom lenses inExamples 1 to 3.

TABLE 12 Example 1 Example 2 Example 3 Conditional −0.971 −0.975 −1.086Expression (1) Conditional −4.905 −6.807 −8.828 Expression (2)Conditional 2.502 2.857 3.380 Expression (3) Conditional 1.348 1.3701.180 Expression (4) Conditional 1.788 1.828 0.869 Expression (5)

INDUSTRIAL APPLICABILITY

According to the present invention, a small-sized zoom lens capable ofhaving high imaging performance and achieving a high magnificationchange ratio as well as high speed autofocus can be provided by applyinga telephoto system of so-called a telephoto type employing a rear focusmethod to minimize a movement of each of lens groups in zooming andfocusing, and an imaging apparatus including the zoom lens above isprovided. In other words, a zoom lens suitable for a small-sized imagingsystem based on the Micro Four Thirds standard can be provided.

SYMBOL LIST

-   G1 . . . first lens group-   G2 . . . second lens group-   G3 . . . positive lens group C-   G4 . . . negative lens group A-   G5 . . . negative lens group B-   STOP . . . diaphragm

1-7. (canceled)
 8. A zoom lens, composed of: an object side lens grouphaving positive refractive power; and an image focusing side lens grouphaving negative refractive power arranged in order from an object side,wherein, the object side lens group comprises, in order from the objectside: a first lens group having positive refractive power; a second lensgroup having negative refractive power; and a positive lens group Chaving positive refractive power disposed at most image focusing side inthe object side lens group; the image focusing side lens group consistsof, in order from the object side: a negative lens group A havingnegative refractive power; and a negative lens group B having negativerefractive power arranged by facing the negative lens group A across anair distance; wherein the first lens group moves toward the object sidein magnification change from a wide angle end to a telephoto end;wherein the positive lens group C faces the negative lens group A acrossan air distance; wherein the positive lens group C in the object sidelens group and the negative lens group B of the image focusing side lensgroup move along the same trace in magnification change from the wideangle end to the telephoto end; and wherein focusing from infinity to aclose object is achieved by moving just the negative lens group A towardan image focusing side and satisfy conditional expression below:−6.807≦(1−βAt ²)×βBt ²<−4.5 where “βAt”: lateral magnification at atelephoto end of the negative lens group A in infinity focusing and“βBt”: lateral magnification at a telephoto end of the negative lensgroup B in infinity focusing.
 9. The zoom lens according to claim 8,wherein focusing from infinity to a close object further satisfies thefollowing equation:−1.80<β2t<−0.94 where “β2t”: lateral magnification at a telephoto end ofthe second lens group in infinity focusing.
 10. The zoom lens accordingto claim 8, wherein the image focusing side lens group satisfies aconditional expression below:2.1<βrt<3.5 where “βrt”: composite lateral magnification at a telephotoend of the image focusing side lens group in infinity focusing.
 11. Thezoom lens according to claim 8, wherein the negative lens group Asatisfies a conditional expression below:1.05<βAt/βAw<1.45 where “βAt”: lateral magnification at a telephoto endof the negative lens group A in infinity focusing “βAw”: lateralmagnification at a wide angle end of the negative lens group A ininfinity focusing.
 12. The zoom lens according to claim 8, wherein thefirst lens group satisfies a conditional expression below:0.5<fl/√{square root over ((fw×ft))}<2.5 where “fl”: a focal length ofthe first lens group “fw”: a focal length of the zoom lens at the wideangle end “ft”: a focal length of the zoom lens at the telephoto end.13. An imaging apparatus, including: the zoom lens according to claim 8;and an imaging sensor which converts an optical image formed by the zoomlens into an electric signal provided on the image focusing side of thezoom lenses.